Software tools for thermodynamic calculation of mechanically unstable phases from first-principles data

This paper introduces the P^4 software package, a set of software tools that automate the process of vibrational free energy calculation for mechanically unstable phases. The Piecewise Polynomial Potential Partitioning (P^4) method is a recently developed method that tackles the issue of phonon instabilities in solid solutions and compounds. The method efficiently explores the system’s ab-initio energy surface by discrete sampling of local minima, which is combined with a continuous sampling of the vicinity of these local minima via a constrained harmonic lattice dynamic approach. The free energy values obtained by this toolkit can be used in thermodynamic assessments and phase diagram calculations. As a unique advantage, it provides the tool to calculate the vibrational free energy in a wide composition range in alloys. This feature is not practically achievable by relying only on brute-force molecular dynamics or random sampling techniques.